(1) Field of the Invention
This invention relates to a photosensitive material for electrophotography which has a novel laminate structure. More particularly, the invention relates to a photosensitive material for electrophotography which comprises an electrically conductive substrate, an intermediate layer formed on said substrate and a top layer laminated on said intermediate layer, wherein said intermediate layer comprises (A) an organic polymeric photoconductor and (B) an polycyclic aromatic nitro compound at an (A)/(B) weight ratio of from 6/1 to 1/6 and said top layer comprises, incorporated in a binder, (C) phthalocyanine or a phthalocyanine derivative and (B) said polycyclic aromatic nitro compound at a (C)/(B) weight ratio of from 10/5 to 10/25.
(2) Description of the Prior Art
In the art of electrophotography, there is broadly adopted a process comprising charging a photosensitive material provided with a photoconductive layer by corona discharge or the like, exposing the photosensitive material imagewise to actinic rays to form an electrostatic latent image on the surface of the photoconductive layer, applying a developer to the surface of the photoconductive layer to form a toner image corresponding to said electrostatic latent image and transferring said toner image formed on the surface of the photoconductive layer onto a copying paper. In this conventional process, after the transfer of the toner image, the photosensitive material is fed to the cleaning step where the residual toner is removed, and it is then fed to the above-mentioned charging step and subsequent steps again.
An electrophotographic photosensitive material that is used repeatedly in the above-mentioned electrophotographic process is required to have some special properties different from properties required of a photosensitive material of the type where a toner is directly fixed on the photosensitive layer. More specifically, in order to prevent fogging in the repeated copying operation and prolong the life of the photosensitive material, it is necessary that the photosensitive material of the former type should have a relatively quick dark decay (the property that the surface potential of the non-exposed area of the photosensitive layer decays relatively quickly in the dark) and a residual potential as low as negligible (the property that the potential left on the exposed area of the photosensitive layer is as low as negligible). When the residual potential of the photosensitive material is high, it already causes fogging at the transfer step. Further, in this case or in the case where the dark decay speed of the photosensitive material is low, electrostatic charges on the electrostatic image formed on the surface of the photosensitive material or electrostatic charges generated for other reason are left on the surface of the photosensitive material even after the transfer and cleaning steps and they cause fogging at the next cycle of the copying operation. Moreover, if the dark decay speed is low, even after the transfer step, toner particles are electrostatically attracted to the surface of the photosensitive material by a relatively strong attracting force and therefore, the efficiency of transfer of the toner to a copying paper is relatively low and the surface of the photosensitive material must be wiped strongly to remove the residual toner from the surface of the photosensitive material. As a result, the surface of the photosensitive material is readily and quickly damaged and the life of the photosensitive material is shortened.
The photosensitive material of this repeatedly used type is also required to have a highly enhanced mechanical, electric or chemical durability. Namely, since the photosensitive material of this type undergoes repeatedly the discharge or irradiation treatment and receives repeatedly friction with a magnetic brush or cleaning member, the photoconductive layer of the photosensitive material is readily mechanically damaged or electrically or chemically deteriorated. Moreover, such a trouble as peeling of the photoconductive layer from the electrically conductive substrate is readily caused while the photosensitive material is being used.
As the substance for forming a photoconductive layer of a photosensitive material, there are known various organic and inorganic photoconductors. Among these known photoconductors, phthalocyanine and phthalocyanine derivatives have been noted as substances valuable for manufacture of photosensitive materials for electrophotography because their chemical and electric durabilities are excellent and they are easily available and cheap.
Photosensitive materials for electrophotography including phthalocyanine or its derivative as a photoconductor, however, fail to satisfy the foregoing requirements sufficiently. For example, a photosensitive material comprising a photoconductive layer composed of a dispersion of phthalocyanine or its derivative in an electrically insulating binder, which is formed on an electrically conductive substrate, is still defective in that the value of the surface potential at the charging step is generally low, the rising speed of the surface potential is low, the residual potential at the exposure step is still at a level that cannot be neglected and the speed of reduction of the potential in the non-exposed area, namely the dark decay speed, is low.